In the field of narrow stripe semiconductor lasers, the alignment of an optical fiber with the edge emitting region of a semiconductor laser is a difficult labor intensive process because of the small dimensions and extremely tight tolerances in fiber-to-laser position. The procedure described in this invention greatly simplifies the process of fiber alignment to an edge emitting narrow stripe laser.
In the alignment of an optical fiber to another optical element, there are five degrees of freedom (three position coordinates and 2 angles). In the prior art, fiber alignment aides were devised to reduce the number of degrees of freedom. One class of aides consists of etched V-shaped grooves in a single-crystal silicon substrate into which a fiber is placed. The optical element is aligned to the groove and thus, to the fiber.
In the prior art Boivin introduced the use of anisotropically etched grooves in silicon for fiber positioning. In his article "Thin-Film Laser-to-Fiber Coupler", Applied Optics, February 1974, Vol.13, No.2, P391-395, he described a laser-to-fiber coupler in which a thin film waveguide is used to focus an incoming laser beam (not restricted to semiconductor lasers) into the core of a fiber. The laser beam is first coupled into a thin-film waveguide by means of a grating coupler and the beam is focused by a thin film lens. The focused beam emerges from the waveguide to enter the core of the fiber. Accurate alignment of the fiber to the waveguide is achieved by supporting the fiber in a V-channel produced by crystallographic etching of the single-crystal silicon substrate on which the waveguide is fabricated.
The prior art also shows an incoming light carrying fiber being supported in a preferentially etched V-groove at the edge of a silicon substrate so that the fiber core feeds light directly into a thin-film channel waveguide deposited onto a SiO.sub.2 layer at the surface of the silicon substrate (See Boyd & Sriram, "Optical Coupling From Fibers to Channel Waveguides Formed on Silicon", Applied Optics, Mar. 15, 1978, Vol.17, No.6, P895-8).
For the previous two examples, the optical element is fabricated on the same silicon substrate into which the V-grooves are etched. The critical alignment of the groove to the optical element is accomplished by photolithography, a technique capable of positional accuracy routinely better than 1 micron.
For a second type of alignment system, the silicon substrate and the optical element are separate pieces. Such is the case for elements which are not fabricated from silicon or thin films deposited on silicon (LiNbO.sub.3 waveguides and semiconductor lasers).
In a flip-chip approach of the prior art, the optic fiber is positioned such that its core is just above the surface of a silicon wafer by placing the fibers in anisotropically etched V-grooves. This assembly is shown by Hsu and Milton in a report "Flip-Chip Approach to Endfire Coupling between Single-Mode Optical Fibers and Channel Waveguides", Electronic Letters, Aug. 5, 1976, Vol.12, No.16, P404-5. A LiNbO.sub.3 channel waveguide is also mounted on the silicon wafer by flip-chip procedure. Before being glued down, the channel waveguide is aligned with the fiber by viewing through a microscope.
In the previous prior art example, there is a difficult, labor intensive alignment. The optical element and etched silicon substrate must be viewed under a microscope and physically positioning one with respect to the other using a micro-manipulator. In the flip-chip approach it is not possible to see the critical portion of the optical element during this alignment. The present invention is directed toward the elimination of this alignment in fiber to semiconductor laser coupling.
The present invention is directed to the self-aligned coupling of an optical fiber to a semiconductor narrow stripe laser or LED. The narrow stripe laser or LED may be any of the several narrow stripe types including the TJS (transverse junction stripe) type, the buried heterostructure stripe laser type, and the proton bombarded narrow stripe type. The semiconductor laser may have a GaAs substrate and may have thin-film epitaxial layers of Al.sub.1-x Ga.sub.x As and Al.sub.1-y Ga.sub.y As. One of the epitaxial layers is an active light emitting layer, the light being edge emitted through a processed laser mirror facet in the region defined by the laser stripe. The narrow stripe emitting spot in the active layer is generally less than 6 microns wide and the present invention is directed to the self-aligned coupling of that emitting spot to the optic fiber core which core is approximately 5-8 micron diameter.